As a method for joining metal members to each other, friction stir welding (FSW) is known (refer to Documents 1 to 9 (Japanese Published Unexamined Patent Applications published by the Japan Patent Office) listed below). According to friction stir welding, a rotating tool is moved along a butting portion between metal members while the rotating tool is rotated, and by plasticizing and fluidizing the metal of the butting portion by frictional heat of the rotating tool and the metal members, the metal members are joined in solid phase. The rotating tool is generally formed by providing a stirring pin (probe) in a projecting manner on a lower end surface of a shoulder having a columnar shape.
Document:    1. Japanese Published Unexamined Patent Application No. H11-342481    2. Japanese Published Unexamined Patent Application No. 2004-358535    3. Japanese Published Unexamined Patent Application No. 2005-131666    4. Japanese Published Unexamined Patent Application No. 2003-164980    5. Japanese Published Unexamined Patent Application No. 2002-248582    6. Japanese Published Unexamined Patent Application No. 2002-1551    7. Japanese Published Unexamined Patent Application No. 2005-66669    8. Japanese Published Unexamined Patent Application No. 2004-154798    9. Japanese Published Unexamined Patent Application No. 2003-326374
When the thicknesses of the metal members to be joined are larger than the length of the stirring pin of the rotating tool, after friction stir welding is performed from the surface side of the metal members, friction stir welding may be performed from the back surface side (for example, refer to Documents 1 to 3). By increasing the length of the stirring pin, the metal members can be joined by performing friction stir welding from only the surface side, however, the load on a drive means of the friction stir welding machine increases, so that if the existing friction stir welding machine cannot cope with this, the friction stir welding machine must be altered or a large-sized friction stir welding machine must be introduced.
When a butting portion between the metal members is friction stir welded, as described in Document 2, a so-called tunnel-like defect may occur along the traveling direction of the rotating tool. There are some cases where the tunnel-like defect does not affect the quality of the joined portion, however, when high airtightness and water-tightness are required, the tunnel-like defect is divided to be discontinuous. Document 2 discloses a technique for joining metal members by forming a depression portion on a butting portion between the metal members by butting the metal members having step portions on their end surfaces and performing friction stir welding a butting portion between a connecting plate fitted in the depression portion and the metal members, and a technique is also disclosed in which the tunnel-like defect is divided to be discontinuous by moving the rotating tool across the butting portion after friction stir welding the butting portion between the metal members.
Documents 4 and 5 disclose a technique for preliminarily joining a butting portion between metal members to be joined before they are primarily welded by friction stir welding, a technique for disposing tab members (splint member) on both sides of the butting portion between the metal members and providing a start position or an end position of friction stir welding for primary welding on the tab members, and a technique for joining the tab members to the metal members before the butting portion between the metal members is primarily welded, etc.
Document 6 discloses a technique for preliminarily joining a butting portion between metal members to be joined by friction stir welding before the butting portion is primarily welded by friction stir welding, and Document 7 discloses a technique for providing a start position or an end position of friction stir welding for primary welding on tab members disposed on both sides of the butting portion between the metal members, and a technique for joining the tab members to the metal members by friction stir welding before the butting portion between the metal members is primarily welded, etc.
Document 7 and Document 8, etc., disclose a technique for providing a start position or an end position of friction stir welding on a tab member disposed lateral to the butting portion between metal members to be joined.
Document 9 discloses a friction stir welding method in which a pilot bore is made at a start position of friction stir welding and friction stir welding is started from this pilot bore.
Friction stir welding conditions such as the rotating speed and feed rate (movement speed) of the rotating tool are appropriately set according to the materials and thicknesses of the metal members to be friction stir welded, and various friction stir welding conditions are disclosed in Documents 2, 3, 5, 6, and 9, and in all of these documents, the rotating speed of the rotating tool is maintained constant and the rotating tool is moved at a constant feed rate from the time when the rotating tool is inserted (pressed) into the start position to the time when the rotating tool is withdrawn from the end position.